Material identification using a remotely located sensor is used for a number of purposes, for example detection, identification and classification of objects within a scene, and applications, for characterization of urban areas, search and rescue, differentiating combatant forces, and detection of attempts at camouflage, denial and deception. It is based on the general principle that the observed spectral radiance of any given target will vary based on, among other things, the type of the material of which the surface of the target is made. Different materials absorb, reflect and emit differently, depending on wavelength. The spectral radiance of a target—the surface of a particular object within a scene—from a given angle or direction can be measured using various types of sensors, depending on the wavelengths of interest. Based on the measured or observed spectral radiance it is possible to determine the material of which the surface is made.
Several types of remote sensors and imaging modalities have been used to generate image sets containing both spectral and spatial information of real scenes for purposes of detection, identification or classification of objects within the scene. Electro-optical sensors are typically single band, multispectral or hyperspectral. A multispectral sensor detects and records radiation in a limited number of bands that are typically fairly wide, for example in the red, blue, green, and near-infrared bands of the spectrum. A hyperspectral sensor detects radiation in a large number of contiguous bands, typically throughout the visible and near-infrared regions of the spectrum. Other types of imaging modalities, for example, synthetic aperture radar (SAR), typically operate only in a single band. The sensors are typically (but do not have to be) placed in satellites or aircraft and acquire images of portions of the surface of the earth during flyovers at relatively high altitudes. However, it is possible for the sensors to be placed on the ground.
Each “image”—also called “imagery” or “image set”—of a scene generated by such a sensor comprises spatial information, typically in two dimensions. It also contains spectral radiance information, which would include the radiance of at least one predetermined band of wavelengths that the sensor can detect. The material of which at least the surface of an object within the scene is made—the “target”—is identified by selecting within the image the pixels comprising the target and evaluating the spectral radiance to develop a spectral signature for that target that can be compared to known spectral signatures of various materials. In automatic material identification a specially programmed computer is used to process image data from a remote sensor and other data to identify the material of the target.
The spectral radiance—radiance at a given wavelength or band—for any given target in a scene will depend on the material of which the target is composed (the “target material”), as well as the spectrum and angle of irradiation being reflected by the target, the atmospheric conditions through which both the illuminating irradiation and the reflected radiation travels, and the spectrum of any emitted radiation. In order to make the identification, measured spectral radiance is typically transformed to an estimated reflectance. Reflectance is the ratio of the measured radiance from an object divided by the radiance reflected by a 100% Lambertian reflector. When using images of real targets, reflectances are estimated by taking into account relevant environmental conditions, such as the radiation source and atmosphere, under which the imagery was acquired.
The way in which a surface reflects or emits radiation can be generally categorized as either Lambertian or non-Lambertian. A Lambertian surface scatters electromagnetic radiation equally in all directions, without regard to the direction of illumination. Thus, its reflectance is generally isotropic or the same in all directions. A non-Lambertian surface does not scatter incident electromagnetic radiation equally in all directions. Examples of non-Lambertian surfaces include those that are backscattering, meaning that the light scatters predominantly toward the illumination source; forward scattering, meaning scattering predominantly in directions away from the illumination source; and specular, meaning reflecting the illumination source like a mirror. Many man-made objects or targets exhibit non-Lambertian reflectance.